1. Technical Field
This invention relates to mesostructured, amorphous aluminum oxide materials with three-dimensionally, randomly connected framework mesopores and methods for synthesizing these materials and the utilization of the said materials. In particular, the present invention relates to such materials where the mesoporous structure is formed via a reorganization process by heating a homogeneous synthesis mixture consisting of alumina species and organic pore-forming agents. The present invention further relates to processes using the said materials as catalysts and/or catalyst supports and adsorbents, particularly in refinery and petrochemical processes.
2. Description of Related Art
Zeolites are highly crystalline microporous aluminosilicates widely used in industry, particularly in petroleum refining and petrochemical processing. Zeolites and some other molecular sieves, e.g. aluminophosphates and pillared clays, have rigorously uniform pore sizes. Depending upon the pore size distribution, porous solid materials are classified as microporous (pore sizes<2 nm), mesoporous (pore sizes between 2 and 50 nm) and macroporous (pore sizes>50 nm).
Many inorganic porous materials are synthesized based on the interaction of organic templates and inorganic species. Individual organic molecules have been used for the templating of micropores for about 40 years. The organic template normally has three functions: filling voids, balancing charges, and stabilizing structural building units. The templating mechanisms and crystallization of zeolites have been documented (Barrer et al., Zeolites, 1, 130–140 (1981); Davis et al., Nature, 331, 698–699 (1988); Davis et al., Chem. Mater., 4, 756–768 (1992) and Gies et al., Zeolites, 12, 42–49 (1992); Hearmon et al., Zeolites, 10, 608–611 (1990)).
One drawback to the use of zeolites as catalysts is that their relatively small pore sizes prevent some bulky, important chemicals from entering the pores of the zeolite and being chemically converted. Thus, there is an increasing demand for novel, mesoporous materials, particularly those that can be used as catalysts, catalyst carriers, or adsorbents.
The use of surfactants, such as long chain, quaternary alkyl ammonium cations, is known to be successful in templating mesoporous materials, such as mesoporous silica and aluminosilicate (cf U.S. Pat. No. 5,098,684 and 5,102,643, the contents of each of which are incorporated by reference herein). Further studies have proposed a pore formation mechanism for these reactions, which involve strong electrostatic interactions and ion pairing between surfactants and anionic silicate species (Beck et al., J. Am. Chem. Soc., 114, 10834–10843 (1992)). This concept has been further developed with the suggestion of four complementary synthesis pathways, implying that both cationic and anionic surfactants can be used. (Huo et al., “Generalized Synthesis of Periodic Surfactants/Inorganic Composite Materials”, Nature, 368, 317–321 (1994).) In addition, one study recently used neutral primary amines or polyethylene oxide to form micelles, which interact with inorganic species via hydrogen bonding (Tanev et al., “A Neutral Templating Route to Mesoporous Molecular Sieves”, Science, Vol. 267, 865–867, (1995)). In all the above methods, surfactants are used to form micelles, which are then capable of templating mesopores.
More recently, the use of small, inexpensive organic molecules as templates has been disclosed to synthesize three-dimensional and stable microporous and mesoporous silicates in the absence of any surfactants (cf WO 00/15551 and U.S. Pat. No. 6,358,486, the contents of each of which are incorporated by reference herein). In the process of mesopore formation, no micelles were formed, but organic aggregates of small template molecules formed in a homogeneous, inorganic matrix upon heating as described by Jansen, et al., “A New Templating Method for Three-Dimensional Mesopore Networks”, Chem. Commun., 713–714 (2001).
Previous methods of synthesizing mesoporous aluminum oxides have employed neutral polyethylene oxides as templates (U.S. Pat. No. 5,622,684 and 6,027,706; Bagshaw et al., Science, vol. 269, 1242–1244 (1995)); carboxylic acids as templates, where the pore sizes of the resulting materials could not be adjusted (U.S. Pat. No. 5,863,515; Vaudry, et al., “Synthesis of Pure Alumina Mesoporous Materials”, Chem. Mater. 8, 1451–1464 (1996)); and a surfactant-assisted synthesis using chelating agents to control hydrolysis and condensation (Cabrera et al., “Surfactant-Assisted Synthesis of Mesoporous Alumina Showing Continuously Adjustable Pore Sizes”, Adv. Mater. vol. 11, No. 5, 379–381 (1999)). However, all these methods are based on the self-assembly of surfactants to form micelles, and it is often difficult to adjust the porosity of the resulting materials.
Accordingly, there remains a need for improved inorganic aluminum materials, such as mesoporous aluminum oxides, which can be used as catalysts, catalyst supports, or adsorbent materials, that possess thermal-stable, three-dimensional pore systems.
There also remains a need for new methods to synthesize mesoporous aluminum oxides with highly thermal-stable, three-dimensional pore systems, where the methods are both economical and permit the adjustment of mesopore sizes.